On 03/21/2016 04:24 PM, Simon Leinen wrote:
> Works from here:

Thanks for testing!

>> Is there anything I can do on my side to diagnose this?
> 
> traceroute/traceroute6?
> 
> You seem to have a public IPv6 address and/or an IPv6 default route,
> because otherwise telnet and other programs wouldn't try to connect
> over IPv6. 

Indeed, I'm on MIT CSAIL's network.

> But maybe your IPv6 connectivity is broken/blocked by
> firewall/nonexistent.  Or, if IPv6 generally works for you, there may
> be a routing problem further "upstream".  As I said, traceroute{,6}
> might help.  What is your IPv6 address? There are some IPv6 peering
> issues/disputes that may cause disconnectivity between some parts of
> the IPv6 Internet, maybe you are being hit by this.  I see dx.doi.org
> behind Cogent, who are involved in these disputes.

Interesting, thanks for the background info! I'm raising the issue with MIT's network admins.

> While the routing issue certainly isn't an Emacs problem, it would be
> nice if Emacs would notice the issue and quickly fall back to IPv4, as
> per the recommendations in RFC 6555.  I assume IPv4 works for you?
> i.e. telnet -4 dx.doi.org 80

telnet -4 works fine indeed. It also works fine in Firefox. Could Emacs use the same tricks as Firefox and fall back?

Clément.